When you are only a single person or a couple, most people would think that they only need a small food processor that meets their modest needs. This is true if you don't have company much or if you don't do much entertaining. A small 3 or 4 cup processor might be perfect for everyday use.
However, if you do like to entertain or have a lot of family members that come for the holidays, it might be a good idea to invest in a food processor that is a little larger. When you use a small processor that has a smaller motor wattage, then trying to prepare large meals for many people could cause it to burnout prematurely. So, that is something to consider when making your choice.
If you do have a large family to prepare meals for everyday, then going with a larger food processor is really your only option. It should be at least a 10 to 14 cup size so you won't have to do more than one batch of processing at a time. A larger motor ensures you can do this without causing damage. A large food chute is also essential so you don't have to do smaller pieces at a time.
The best way to decide what brand and size processor you really need to to visit a page like Luxury of the Pharaohs in order to make side by side comparisons. You can read about why certain brands and models might best suit the needs of your kitchen and be able to pick the best one that will last you the longest.